For some raason I doubt this, but if I go any further a wiki page will pop up and prove me wrong.

The snipers are pretty simple.
With SP:
Intervention/Barret=1 shot to the head/torso and 2 shots to the arms/legs/lower waist area.
M21/WA2000=1 shot to the chest/head and 2 shots to arms/legs/stomach area.
That's with stopping power,remember though..the WA2000 has no damage increase with stopping power(though if they have already taken damage it will have a higher chance of killing with stopping power if you hit an area that usually wouldn't be a 1 shot kill,but that isn't too common..so a good rule is to not use SP with the WA200).
Without SP:
Intervention/Barret=1 shot to the head/chest,2 shots everywhere else.
WA2000=exactly the same as without stopping power,1 shot to head/chest 2 shots everywhere else-this makes it the best sniper rifle to use without stopping power.
M21=headshots are 1 shot kills,everything else is 2-3,you have to spray to kill anything basically..not recommended.
So yeah,with SP they're exactly the same..you can look it up if you don't believe me.
Silencer with SP is pretty much the same as without SP or a silencer.